In a liquid crystal display, a backlight control circuit is used which controls light emitting diodes (LEDs) to illuminate from the back side of a liquid crystal screen, so that a user can observe an image from the front side of the liquid crystal screen.
In early days, LED backlight is used only in a small size screen, which does not require high backlight brightness. Therefore, the LEDs can be connected all in series or all in parallel. FIG. 1 shows a prior art circuit wherein all LEDs are connected in series. As shown in the figure, a backlight control circuit 1 comprises a backlight control integrated circuit 10 which includes an input terminal and an output terminal, wherein the input terminal is connected with an input capacitor Cin to receive an input voltage Vin, and the output terminal is connected with an output capacitor Cout to provide an output voltage Vout. (Besides the backlight control integrated circuit 10 and the two above-mentioned capacitors, other devices irrelevant to the spirit of the present invention, such as magnetic devices, are omitted for simplicity.)
The backlight control integrated circuit 10 provides output voltage Vout to a plurality of LEDs L1-LN connected in series, and the output voltage Vout is provided via a voltage supply circuit 11 according to a signal 15 which is outputted from an error amplifier circuit 13. A resistor R is provided on a path of the LEDs connected in series, and a voltage at a node Vsense1 is compared with a reference voltage Vref to check whether a current through the path satisfies a predetermined condition. If the current is lower than a predetermined value and the voltage at the node Vsense1 decreases, the error amplifier circuit 13 sends the signal 15 to the voltage supply circuit 11 to pull up the output voltage Vout, so that the current flowing through the LEDs increases. Additionally, to avoid the voltage supply circuit 11 from unlimitedly increasing the output voltage Vout (for example, when the error amplifier circuit 13 malfunctions, or when the path of the LEDs is open), an over voltage protection circuit 12 is provided in the backlight control integrated circuit 10, which detects the output voltage Vout and sends a signal to stop the voltage supply circuit 11 from increasing Vout if the output voltage Vout is excessively high. (Depending on circuit design, the voltage supply can be totally stopped, or kept at an upper limit value. The latter is more popular in a backlight control circuit.)
FIG. 2 shows a typical structure of an over voltage protection circuit 12, wherein the output voltage Vout is monitored by comparing the voltage at the node Vsense2 with a reference voltage Vovp. The result of comparison determines a signal for controlling the voltage supply circuit 11.
Referring to FIG. 3, it shows a conventional backlight control circuit with LEDs all connected in parallel. As shown in the figure, a backlight control circuit 2 comprises a backlight control integrated circuit 20, wherein the currents passing through LEDs L1-LN are respectively controlled by the current sources CS1-CSN. The backlight control integrated circuit 20 comprises a minimum voltage selection circuit 21 which chooses a lowest voltage value among all voltages at cathode ends of the LEDs L1-LN, and the error amplifier circuit 13 compares the lowest voltage value with a reference voltage to generate a signal controlling the voltage supply circuit 11. Thus, the output voltage Vout is under control so that all current source circuits are provided with sufficient operating voltage for normal operation, and all LEDs can illuminate normally thereby.
Similarly, the backlight control integrated circuit 20 can further comprise an over voltage protection circuit 12 as the one described above.
The number of LEDs that are allowed to be connected all in series or all in parallel in the above conventional arrangements is limited, and naturally this leads to connecting the LEDs partially in series and partially in parallel (series-parallel connection). FIG. 4 shows a prior art arrangement of such series-parallel connection in which the backlight control integrated circuit 10 shown in FIG. 1 is employed to provide voltage to a series-parallel connection circuit of LEDs. However, it only checks the current on the path of LEDs L1-LN but does not check those on the other paths.
Another prior art arrangement is shown in FIG. 5 which employs the backlight control integrated circuit 20 shown in FIG. 3 to compose a series-parallel connection circuit for LEDs.
In the above circuits shown in FIGS. 1, 4, and 5, the larger the number of the series-connected LEDs is, the higher the required output voltage Vout is. Correspondingly, a higher voltage rating capacitor is required for the output capacitor, which will increase the total cost of the backlight control circuit.